1. Field of the Invention
The present invention relates generally to the field of thermal regeneration of water-contaminated molecular sieve material used in oxygen concentrators. More specifically, the present invention discloses a method and apparatus for regenerating molecular sieve material using a combination of heat and a dry sweep gas.
2. Statement of the Problem
Oxygen concentrators are widely used for medical purposes to produce concentrated oxygen by removing nitrogen from air. The pressure swing adsorption process used in virtually all present day oxygen concentrators is capable of producing up to 95.6% oxygen. There are currently estimated to be in excess of 500,000 of these systems in use.
A molecular sieve material, such as zeolite, is employed by the oxygen concentrator to capture the nitrogen molecules from the air. A typical oxygen concentrator contains up to ten pounds of molecular sieve material, at a cost of approximately $4.00 to $10.00 per pound. However, molecular sieves are also capable of preferentially adsorbing other constituents found in air, such as water vapor and carbon dioxide. In particular, they have an extremely high affinity for holding water molecules. When water vapor is allowed to accumulate in the molecular sieve material, the pores and capture surfaces are blocked for nitrogen removal. The concentrator thus fails to perform its intended purpose.
In commercial drying applications, the molecular sieve material must have a water content below 10% by weight to produce air with a -40.degree. F. dew point, and to below 3% by weight to produce air with a -100.degree. F. dew point. In contrast, in order for an oxygen concentrator to function efficiently, the molecular sieve material should have a water content below 1.5% by weight, and preferably below 1% by weight. A water content of 2% by weight will severely hamper concentrator performance. A 5% water content will render it useless. For example, a sieve exposed to 20% relative humidity at 80.degree. F. will quickly accumulate a water content equal to 20% of its total weight and render it useless as an oxygen concentrator. Even commercially dry air having a dew point of -40.degree. F. is capable of contaminating molecular sieve material to 10% of its weight in water. Thus, water contamination is a major problem in molecular sieve materials used for concentrating oxygen to a much greater degree than in other commercial drying applications.
During normal operation of an oxygen concentrator, most atmospheric water vapor is ejected back into the atmosphere by the pressure swing operation of the concentrator. Water contamination of the molecular sieve material within the concentrator can result from valve failure, or other leakage, or if the molecular sieve material is accidentally exposed to atmospheric air for a period of time.
The prior art contains other examples of thermal regeneration of molecular sieve materials, including the following:
______________________________________ Inventor U.S. Pat. No. Issue Date ______________________________________ Sircar, et al. 4,971,606 Nov. 20, 1990 Sircar 4,784,672 Nov. 15, 1988 Francis, et al. 3,756,961 Sept. 4, 1973 ______________________________________
Sircar, et al. U.S. Pat. No. 4,971,606, discloses a thermal regeneration system using a combination of high temperature and dry sweep gases (e.g. air or nitrogen) to remove water absorbed by zeolite adsorbents. A hot regeneration gas is passed through a bed of the adsorbent at a sufficiently high flow rate such that the residence time and reaction of the desorbed components in the adsorbent bed are minimized. Residence times of less than one second are discussed.
Sircar U.S. Pat. No. 4,784,672 discloses a process for regeneration of adsorbents used in pretreatment of landfill gas. One of the pretreatment sections is a layer of molecular sieve zeolite that is regenerated by a flow of hot regeneration gas.
Francis, et al., disclose a process for regenerating a bed of coke-containing zeolitic molecular sieves by continuously passing a closed-loop flow of hot, oxygen-containing inert gas through the molecular sieve bed. Water in the circulating gas stream is maintained below a predetermined concentration.
3. Solution to the Problem
None of the prior art references uncovered in the search show a method and apparatus for thermal regeneration of molecular sieve materials using a combination of heat and a dry sweep gas to achieve the extremely low water-content tolerances required by medical oxygen concentrators. In addition, the dual-chamber desiccant air dryer employed in the present invention is neither taught nor suggested by the prior art for this field of use.